Alarm system



E2 Sheets-Sheet 1 A. H. FAULKNER ALARM SYSTEM SS 55,3@ QE Filed Feb. 17,1956

De'c. 17, 1957 A, H. FAULKNER ALARM SYSTEM I2 Sheets-Sheet 2 Filed Feb. 17, 1956 United States Patent C ALARM SYSTEM,

Alfred H. Faulkner,y Chicago, Ill., assigner to General Telephone Laboratories, Incorporated, a corporation of Delaware Application February17, 1956, ScrialNo. 566,281

13- Claims., (Clt 340-.-213) The present.inventionrelates to alarm systems` and particularly to electronically controlled circuits kfor such systems.

Au object of the invention is to provide `an improved alarm system having a central station and a plurality of control `stations connected thereto. in` a normally closed circuit arrangement -wherein an alarm is rendered at the centralstation-in response to` an abnormal condition at any one of the controlk stations and wherein anotherr alarm is rendered at the central station in response to an interruption of any one, of the connections between the central s tationand the control stations.

Another object of the. invention is to provide in an alarm system ofthe closedA circuit type improved operating circuits having low power. requirements.

Aturther yobject of the; invention is fto provide in an alarm system. ofthe closed circuit typeimproved operating circuits utilizing transistors and diodes to perform the control operations therein.

A further object of the invention is to` provide in an alarm system including a plurality of linel circuits respectively associatedwith :the controlstations thereof 4an improved transistorized linel circuit operated in a normal statein response toa normal condition at the corresponding, control station andoperated in a first alarm state, in;

response to an abnormal conditionV at the corresponding control station and operated in asecond alarm state in responsetoaninterruption in the connection between, the line circuiti and. thev corresponding controlV station.

additional object of theinvention is` to; provide in. anpalarm system. linevcircuirt controlmeans for controlling theline circuits thereof through any of their operating states fin order `to restore t-o the normal operating state` any line circuit operated to an alarm .state and inorder to test the alarm operation of any of the line circuits.

Further features ofthe invention pertain to the particular arrangement of circuit elements in the alarm system, whereby the above outlined objects andV additional operating featuresthereof are` attained.

The invention both as to its organization and` method of operation, taken together with further objects and ad-` vantages, thereof, will be. best understood byV reference tothe following specification taken` in conjunction with they accompanying drawing in whicl'il Figures l and 2, taken together, illustrate in schematic'formt a high: ternperature'alarm systemv and the apparatus `included therein embodying theprincipleslof thepresent invention'.

Section 1.-Tha high temperature alarm system cludingthetemperature control station-s` 20 and 30,.'l AA pluralityl of twoconductor` control lines, including the,

CODIQUiHS- 246 M4248, respectively conneettheitemperaturefcontrol stations,20, etc.,to the centralsta-tionl.,

In the-*temperature control stations 20,-etc., the conductorsA 2,@ l 7,074 Patented, Dec.V 17,y 1957 ICC of the corresponding control` lines 246 etc., are connected together through temperature-sensitive impedance devices, including the temperature-sensitive impedance devices 250 and 255,so that the impedance of the path completed thereover is varied in accordance with changes in the temperature at the corresponding temperature control` station. In the central station 10, the control lines 2456, etc., areterminated respectively in line circuits, including the line circuits 220 and 220A, which are selectively operated'in accordance with the impedance established in the path extending through the conductors of the corresponding control line. In the central station 10, the line circuits 220, etc., are commonly connected to power equipment which supplies power thereto and to alarm equipment which may be operated in response to signals received from any one of said line circuits.

Referring specifically to Fig. l, the terminal equipment located inthe central station 10 includes a power supply adapted to convert alternating current power to direct current power for operating the fire alarm system, a grounded line detector unit 110, a trouble alarm controller responsive to trouble signals returned from theline circuits 220etc., a temperature alarm controller responsive to temperature alarm signals, returned from the line circuits 220, etc., a voltage regulator for distributing a constant voltagervariable current to the line circuits, a temperature alarm 187, a trouble alarm 190, a test switch for testing the operationl of the line.cireuits. 220, etc., and a asher unit; .160, for supplying` a hashing signal to an attendant at the` central station 10r when thev test switch 180 is -operated into a test position. Figure 2 illustrates two of the line circuits 220 and 220A, which are identical in structure and which are connected respectively through line circuit control switches, including the control switches 240 and 240A, to the corresponding control lines 246, etc. In the temperature control stations 20, etc., the temperature sensitive impedance devices 250 etc., respectively complete a connection between the conductors of the corresponding control lines and under normal temperature conditions thereat exhibit a high impedance to current ilow therethrough and under high temperature conditions thereat exhibit a low impedance to current flow therethrough.

Section 2.-The equipment of the alarm' system Considering now the equipment included in the high temperature alarm system, the system is designed to operate on 120 Volt D. C. current supplied to power terminals 203 and 204 on the terminal block 202 from, a 120 volt D. C. power source or, alternatively,` from a 120 volt A. C. power source. From a D. C. source, power is applied over the input line 91, including the negative and positive input conductors 91A and 91B, respectively, which are connected directly to negative and positive power conductors 193 and 194 terminated at the power terminals 203 and 204, respectively. From an A. C. source, power is applied over` the input line 92 including the negative and positive input conductors 92A and 92B, respectively, directlypto the primary winding of a step-up transformer T109 located in the power supplyv 100.

The power supply 100, which'is connectedat its output to the power conductors 193 and 194, is of a conventional arrangement and includes in addition, to, the step-up transformer T109, a bridge rectifier provided with diodes D101, D102, D103 and'D104, a lter reactor LIOS bridged by a capacitor C106 in order to minimize the peak to peak value of the ripple in the. output voltage of the rectier, and a pair of parallel coupled filter capacitors C107 `and C108. A pilot lamp LP111 of theneon discharge type is connected in series with. a resistor 11-2 across` the power conductors. 193-and 194 and is illuminated when power is being applied to the power conductors 193 and 194 thereby indicating the operative state of the alarm system. In addition, `a grounded line detector unit 110 including resistors 113, 114, and 115 and line conductor ground lamps LP116 and LP117, of the neon discharge type, are connected across the conductors 193 and 194 in order to render an indication when any line conductor in the system is grounded as will be described in detail hereinafter.

The trouble alarm controller 130 is connected via conductor 195 to a terminal 205 on the terminal block 202 and includes the transistor TR131 arranged to amplify trouble alarm signals received over conductor 195 and to operate a trouble relay R136 for lcompleting an operating circuit through the trouble alarm 190 which may be of the buzzer type. The temperature alarm controller 140 is connected via conductor 196 to a terminal 206 on the terminal block 202 and includes theA transistor TR141 arranged to amplify temperature alarm signals received over conductor 196 and to operate a temperature relay R146 for completing anv operating circuit through temperature alarm 187 which may be of the ring type.

The voltage regulator 150 supplies a constant-voltage current via conductor 197 to the terminal 207 on the terminal block 202, and the regulator comprises a load resistor 159 for establishing a Xed voltage on the terminal 207 and transistors TR151, TR152, TR153 and TR154 connected therein for varying the current ow in conductor 197 in accordance with current demands made at the terminal 207.

The test switch 180 is a three position switch, and has associated therewith the temperature alarm 187, the trouble alarm 190, a temperature test alarm lamp LP189 of the neon discharge type and a trouble alarm test lamp LP192 of the neon discharge type. In its normal position the test switch 180 connects the temperature alarm 187 and the trouble alarm 190 to the respective operating circuits therefor; in a temperature alarm test position the test switch 180 disconnects the temperature alarm 187 from the corresponding one of the above mentioned operating circuits and connects the temperature alarm test lamp LP189 to that circuit and also completes a circuit for operating the flasher unit 160; and in a trouble alarm test position the test switch 180 disconnects the trouble alarm 190 from the other of the above mentioned operating circuits and connects the trouble alarm test lamp LP192 to that circuit and also completes a circuit for operating the lasher unit 160.

The asher unit 160 illustrated in Figure l includes transistors TR161 and TR162 connected in a relaxation oscillator circuit to provide a square wave output voltage, and a test lamp LP171 of the neon discharge type which is operated by the square wave output voltage.

Referring now to Figure 2, the line circuits 220 and 220A shown in detail therein are identical; and the line circuit 220 includes the transistors TR221, TR222 and TR223, a temperature alarm lamp LP227 and a trouble alarm lamp LP230, both of the neon discharge type. The high temperature alarm system operates on a closed circuit basis so that normally a circuit is completed from the positive voltage on the terminal 204 to the negative voltage on the terminal 203 through the line circuit 220, the associated line circuit control switch 240, the control line 246 and the temperature-sensitive impedance device 250 located at the temperature control station 20. The line circuit control switch 240 is accessible to an attendant at the central station and is operable from a normal position into a temperature alarm test position and from the normal position through a line cut-off position and into a trouble alarm test position. In the normalposition the line circuit control switch 240 completes the above traced circuit through the temperature-sensitive impedance device 250 in thetemperature control station and between the terminals 203 and 204; in the temperature alarm test position the line control circuit 240 completes a connection between the conductors 246A and 246B of the control line 246 so that the temperaturesensitive device is short circuited and an effective low impedance condition therefor is displayed to the line circuit 220; in the line cut-olf condition the normal connection at the switch 240 to the conductors 246A and 246B is interrupted; and in the trouble alarm test position, a load resistor 247, of an impedance comparable to the normal temperature impedance of the temperaturesensitive impedance device 250, is connected to the line circuit 220.

Each of the temperature-sensitive impedance devices 250, etc., located in the temperature control stations 20, etc., may be dilerent in a minor respect as will be pointed out hereinafter, but are alike in the respect that at normal temperatures each exhibits a high impedance to current ow therethrough and at temperatures above a critical normal temperature exhibits a low impedance to current ow therethrough. Specifically, the temperature-sensitive impedance device 250 located in the temperature control station 20 includes a mercury column switch 251 connected across the line conductors 246A and 246B of the control line 246 in a normally open circuit arrangement and a load resistor 252 connected across the line conductors 246A and 246B in parallel with the mercury column switch 251 to establish a high impedance connection between the line conductors. At a predetermined high temperature the mercury column switch 251 is closed so that the load resistor 252 is shunted by the mercury column and a low impedance connection is completed between the line conductors. An alternative embodiment is the temperature-sensitive impedance device 255 of temperature control station 30 which comprises two mercury column switches 256 and 257 arranged in spaced apart relationship in the ternperature control station and a load resistor 258, all connected in parallel and operating substantially as the temperature sensitive impedance device 250 so that at normal temperatures a high impedance connection is established across line conductors 248A and 248B of the control line 248 and at a predetermined high temperature either or both of the mercury column switches 256 and 257 are closed and a low impedance connection is established across the line conductors 248A and 248B.

The operation of the equipment included in the high temperature alarm system will be better understood from the following considerations of: operation under normal temperature conditions; operation when a high temperature condition exists therein; operation when a fault condition exists therein; and operation when any one of a number of test conditions is imposed thereon.

Section 3.-Operation of the high temperature system under normal temperature conditions At the central station 10, the potential across the power conductors 193 and 194, applied either directly from ther D. C. input conductors 91A and 91B or through the power supply from the A. C. input conductors 92A and 92B, is 120 volts, which causes the pilot lamp LP111, to break down and to sustain a current ow therethrough for illuminating the lamp. The failure of the pilot lamp LP111 to illuminate indicates that power is not being applied to the input conductors or, if the A. C. power source is being utilized, that some fault lies in the power supply 100.

The power conductors 193 and 194, the line circuits 220, etc., the conductors of the control lines 246, etc., and the temperature-sensitive impedance devices 250, etc. are normally not grounded so that the potential with reference ground established at the power conductors 193 and 194 and the corresponding terminals 203 and 204 depends upon the distribution of leakage resistance between elements of the system and ground. The system is designed so that in normal use the leakage resstances between the conductors of the control lines 246, etc., extending from the line circuits 220, etc., to the tempera- Y ture control stations 20, etc., and ground are predominant and are balanced with respect to the power conductors 193 and 194 so that in vthe system voltages with respect to ground at these conductors and at their terminals 203 and 204 are, respectively, -60 volts and +60 volts, the voltages with respect to ground on the conductors of the control lines extending to the control stations 20, etc., such, for example, as conductors 246A and 246B of the control line 246 are, respectively, -40 volts and +40 volts, and zero volts with respect to ground is established in the load resistor of each of the temperature-sensive impedance devices 250, etc.

In order to provide power for operating the trouble alarm controller 130, the temperature alarm controller 140 and the voltage regulator 150, a voltage divider, including the resistors 120, 121, 122, 123, 124 and 125, is connected between the power conductors 194 and 193 so that at the junction between resistors 120 and 121 the normal voltage is +50 volts, at the junction between resistors 121 and 122 the normal voltage is +47 volts, at the junction between resistors 122 and 123 the normal voltage is +275 volts, at the junction between resistors 123 and 124 the normal voltage is 48.5 volts, and at the junction between resistors 124 and 125 the normal voltage is -53.5 volts. The resistor 124 is in fact the terminal to terminal resistance of a potentiometer having a variable center tap 126 on which a voltage in the range from 48.5 volts to 53.5 volts may be established.

In the trouble alarm controller 130 the transistor TR131, which is of the PNP junction type, is normally biased non-conductive by +50 volts applied to the emitter electrode thereof from the junction between resistors 120 and 121 and +50.5 volts established the base electrode by the leakage current owing in a path via resistors 133 and 132, the base-collector path of transistor TR131, and the winding of relay R136 from +60 volts on the power conductor 194 to +27.5 volts at the junction between resistors 122 and 123. The current flow through the above traced path is less than .05 ma. so that relay R136 is not operated and the voltage established at the junction between resistors 133 and 132 and on the conductor 195 extending therefrom to the terminal 205 at the terminal block 202 is +50 volts. A diode D134 is connected between the base and emitter electrodes of the transistor TR131 in order to limit the reverse bias therebetween and thereby maintain the leakage current through the base-collector path at a minimum. Another diode, D135, is connected from +47 at the junction between resistors 121 and 122 to the junction between resistors 133 and 132 and conductor 195 in order to establish a minimum voltage on the conductor 195 at approximately +47 volts.

The temperature alarm controller 140 is substantially identical to the trouble alarm controller 130 and is connected to the power conductor 194 and to the junctions between resistors 120 and 121 and between resistors 122 and 123 in the same manner as is the trouble alarm controller 130. Thus, the transistor TR141 therein is normally non-conductive, the relay R146 is not operated and the voltage established on the conductor 196 and on the terminal 206 at the terminal block 202 is +50 volts.

Referring now to the voltage regulator 150, the four transistors TR151 to TR154, inclusive, which are of the PNP junction type, are connected in multiple in a modied grounded collector circuit. The collector electrodes of the transistors TR151 to TR154, inclusive, are connected, respectively, through resistors 155 to 158, inclusive, to -60 volts on the power conductor 193 and the emitter electrodes are all connected through the load resistor 159 to +60 volts on the power conductor 194. The base electrodes of the transistors TR151 to TR154, inclusive, are connected to the variable voltage tap 126 of the potentiometer resistance 124 and the voltage thereon is adjusted so as to render the transistorsA conductive and to permit a small current in the emitter-collector paths thereof such that normally a current of approximately 5 ma. is drawn through the resistor 159, thereby establishing the voltage on the conductor 197, connectedk to the junction between resistor 159 and the emitters of the transistors, at -50 volts.

The test switch 180 in its normal position prepares at contacts 183 a circuit extending from +60 volts on the power conductor 194 via contacts 147 the relay R146 and the temperature alarm 187 to -60 volts on the power conductor 193, and prepares at contacts 185 a circuit extending from +60 volts on the power conductor 194 via contacts 137 of the relay R136 and the trouble alarm 190 to -60 volts on the power conductor 193. In the temperature alarm test position the test switch 180 completes at contacts 181 a circuit for operating the asher unit and opens contacts 183 and closes contacts 182 in order to substitute the series connected resistor 188 and the temperature alarm test lamp LP189 for the temperature alarm 187 in the previously traced circuit. In the trouble alarm test position the test switch. completes at contacts 186 a circuit for operating the asher unit 160 and opens contacts 184 and closes contacts in order to substitute the series connected resistor 191 and the trouble alarm test lampA LP192 for the trouble alarm 190 in the previously traced circuit.

The llasher unit 160, as explained above, is operated only when the test switch 180 is in the temperature alarm test position or the trouble alarm test position.

base electrode of TR161 is connected via a charging\ capacitor C171 to the junction between resistors 167 and 168, and the base electrode of TR162 is connected via a charging capacitor C172 to the junction between resistors 164 and 165. When either of the contacts 181 or 186 is closed -60 volts is applied to the emitters of the transistors TR161 and TR162, whereby, and as will be explained in detail hereinafter, the transistors become alternately conducting so that a substantially square wave voltage signal is developed at the collector electrodes thereof. In order to utilize one of the square wave voltage signals for providing a flashing signal, a voltage divider including resistors 169 and 178 is connected between +60 volts on power conductor 194 and 60 volts on the emitters of TR161 and TR162, and a test position lamp LP171, of the neon discharge type, is connected at one electrode to the junction between resistors 169 and 178 and at the other electrode to the collector of TR162.

The manner in which the test position lamp LP171 is operated will be considered in detail hereinafter with the consideration of the operation of the asher unit 160.

The grounded line detector unit 110 is connected across the power conductors 193 and 194 in order to record a grounded conductor condition in any one of the control lines 246, etc., of the system. Specifically, resistors 113, 114 and 115 are connected in series between the power conductors 193 and 194 so that normally the voltage at the junction between resistors 113 and 114 is -50 volts and the voltage at the junction between resistors 114 and 115 is +50 volts. The positive line conductor ground lamp LP116 is connected at its cathode to the junction between resistors 113 and 114 and at its anode to ground, and the negative line condutcor ground lamp LP117 is connected at its anode to the junction between resistors 114 and 115 and at its cathode to ground, so that under normal conditions neither lamp is illuminated. However,

The transistors TR161 and TR162 therein are of the NPN junction type coupled in a relaxation oscillator circuitA @einen and as zeliplrled'inldetail hereinafter, .in response to .the gflllltiing'ofganyr 011e ofr the..negative.line conductors of the .contfroli lines; 246, sete., normally at .-.40 .volts, ,the negative fgronndfdetectom flarnp LPll'- is illuminated,.; and in `response to: thev grounding :of any onefof.z :thelpositive line conductorsothe control .lines:246, etc., normally at +40 volts, the positive ground detector :lamp LP1116 is illuminated; thereby'renderingf the attendantatfthefcentral AStationdO notice'of'a grounded conductor condition. In ordenata-:prevent the; lamps LB116. and vLP117 from being discharged yby instantaneoussurges.- in the voltage on` eitherfof; the. powenconductors 1,93 and194,-a shuntingv .capacitor G118f=is f connected across the :electrodes f` thev lamp.. LP116 .and a shuntingncapacitor`C11l9.:is connected, across the. electrodes of :ther` lamp LP1'17.

11n; the line-circuits,: such,;fornexample, as the line circuit.; 220,;the.. transistors; TR221, TR222 and TR223 areall.oftheNPNjunctiomtype A yoltagerdixider cornprisinggresistorsl 2253,1235, 'and-5233, connected. between +60 volts on the 'terminal- 204. of thetermi-nal block 202. and -.60 volts on the terminal '203,-of theterminal block 202, establishes a voltage-at tl1e junction. between resistorsA v225,and1235` and the collector of TR222 of approximately +40 volts and a voltagetat the junction between `resistors `233 and-.1235 andthe basefofzTRZZl of" tends from` terminal 204 via resistor 224, contacts 244 ofthe line `circuit control. switch 240, line conductor 246B,y load resistor 252 of the temperature-sensitive impedance device 250, line conductor 246A, `contacts 242 ofthe line circuit control switch 240, resistors 231 and 232, the collector-emitter path of TR221, and resistor 234 to the terminal 203, so that a current of approximately 4 ma. ows therethrough establishing the line conductor 246Bv at +40 volts, the line conductor 246A at -40 volts, the emitter electrode of TR223 at the junction between resistors 231 and 232 at approximately -48 volts,

the base of TR222 and the collector of TR221 at approximately -52 volts and the emitter of TR221 at approximately -53 volts. At this time the emitter of TR222 is at volts so that TR222 is biased non-conductive, by a small, current ilow of approximately .2 ma. in the collector-base path thereof. The +40 volts at the collector of TR222` is applied via resistor 226 to the anode of the temperature alarm lamp LP227 which lamp has its anode connected to `+50 volts on the terminal 206. At the transistor TR223 the collector is connected via resistor 228` to volts on terminal 204, the base is at -50 volts and the emitter is at ,48 volts so that a small current of approximately .2 ma. ows in the vcollectorbase path thereof biasing the transistor non-conductive and establishing the collector thereof at approximately +40 volts. The voltage on the collector of TR223 is applied via resistor 229 to the cathode of the trouble alarm lamp LP230 which has its anode connected to +50 volts at the terminal 20S. As the potential across the electrodes of the lamps LP227 and LP230` is substantially less than the breakdown voltage, these lamps at this time are non-conducting and not illuminated.

Thus, under normal operating conditions and normal temperature conditions, the test switch 180 Aand thefline circuit control switches 240, etc., are in their normal positions, the pilot lamp LP111 is illuminated, the asher unit 160 is inoperative, and the line circuits arein an operativestate. so thatin anyone thereof, such, for example, as the linercircuit` -22`0, the transistor, TR221is conductive --and the transistors .'I'RZZZ-.eindl TR223 ,are`

non-conductive.

-Further operation. .of the circuitsincluded; 4inr'the :high temperature. alarm systemrwilbbe understood:'from the x following consideration: of a high temperature.conditi,on

therein.

Section* .4f-#Operation'ofv the highl temperture Valarm System when a high temperature condition exists therein Yshortcircuited. Thereupon, the,current ow. through the line circuit. 220 and-theline.conductors 246A.and 246B increased from. 4 ma. to :about 13.5 ma., causing, the potentialdrop through resistors 231 and232 to increase so that the base-,electrodeof .the transistor TR222 goes from -52 voltstouapproximately -50 vvolts. At this time,the emitter electrodefof-.the transistor TR222.is at approximately Vn50-.volts and. TR222 is biased conductivev so that aJargecurrent, linitially 9 ma., ilows through the base-emitter; paththereof, and the potentialon the collector elect-rode.` .Swings-from +40 volts to-approximately S0- volts sothatfthe potentialacross resistor 226 and the temperature alarm lamp LP227 `is increased from approximately l;0..volts to lapproximately 1.00 volts, and the lamp Ll227. breakspdown and supports a discharge current ,ofapproximately 2 ma. therethrough, .whereby the temperature alarm-l-arnpLPZZ glows. to'indica'te to .an attendant at .the ,central station that a ,highl vtemperature condition exists inthe'temperature, control station 20.

Normally, `theeurrent flow; through` the resistor 233 isabout .35. ma. ..of;which ,approximately .3 ma. ilows throughtheJbase. of,-,thetransistor TRZZI.; the emitter current ofi-the, .transistor .TR22lfis approximtely 4. ma. so thatthe current ow intoL thecollector electrode is approximately 3.7 .ma. Thereafter, whenthe transistor TR222is rendered conductive and theicollector voltage is at -5.0.vol ts, .theyOI-tage. on the base of, the transistor '1`-R221.cha r1ges' from -10 volts yto -56 volts thereby causing-the.. collector-emitter current of. TR221 to be limited approximately 1 ma.; thereupon the base-emitter current of IR222\ is increased to 12.5 ma. At this time, the transistor TR222, is rfully,conductive and restricts conduction .in .thetransistor TR221 `so that TR221 is substantially non-conductive. Should the high temperature conditionat the temperature control ystation 20 subsidefthereaftenmso that the temperature-sensitive impedance. device '250again exhibits a vhigh impedance characteristicv andthe current ilow in the line conductors is decreased from 13.5l ma.A to 4 ma. the transistor TR222 in Ithe line circuit 220 remains conducting because at this time the collector-emitter current of TR221 is limited to `appl-.eximately l` lmaso thatthe transistor TR222 conducts;` 3, ma.l throughjhe base-emitter path' thereof. In order to protectthel transistor TR222 from false operation .dueto powersurges a shunting capacitor 236- is connected in ,the line-circuit 220.y

The, anode ,of ,the temperature alarm lamp LP227, is maintained `at -450 yolts by the voltage of the terminal 206which is applied, thereto by conductor `296 from the temperature alarm controller 140. When Vthe lamp LP227 ,is illuminated and` draws a 2 ma. current through the=conductor .196 from the tempera-ture alarm controller 140, the transistor'TR141v therein is rendered conductive and the relay R146 is yoperatedt-olclose contacts 1'47 'the circuiefor operating the temperature alarm187. Specifically, in thetempcrature alarm controller 140,12 ma.

current, demand of conductor 196 is supplied .through re- .base electrode ofrthe transistor TRl41 is established at .approximately -47 volts, whereby the transistor is biased path and 4resistor 142. vIn addition current flows in the emitter-collector path of TR141 and the winding of relay R1 46 to 27.5 volts at the junction between resistors 122 and 123 so that relay R146is operated and contacts `147 are closed to complete a circuit extending from +60 volts on the power conductor.194,'contacts 183 of the 'system 'The attendant then checks all of .the temperatfulre alarm lamps in the system andis given notice by the il1uini'n'at-ed condition of the temperature lamp LP227 .th'a'tetjhe high temperature condition exists in the tem-- peratu're control station 20.

` 'Agaim 'considering the conduction of the transistor TR222 in the line circuit 220, as previously mentioned, theI maximum currentiow through the base-emitter path thereof isv l2 .5 ma. and the current flow via the temperature alarmlarrip LP227 and throughthe collector-emitter pathcf TR222 is 2 ma. so that the maximum current ow fromthef emitter electrolleofthe transistor TR222 is approximately 15 ma. This 15 ma., current ow is applied via theterminal i207 of the terminal block 202, to .the conductor 197 extending to the voltage regulator 150. As

previonslymentioned, the 50 volts on the conductor 197 is normallyes'tablished by the 5 ma. current ilowing from the 'power conductor 1,94 through the resistor 159 and the emittercollector paths of the transistors TR151 tol l TR154, inclusive, to the power conductor 193. However, when a high vtemperature condition develops in ony `one of the control stations' 20, etc., the 15 ma. current applied to the conductor 19,7 tends to reduce the normal 5mn. current llo'w lthrough the'resistor '159 whereupon the potential drop through the resistor '159 is reduced thereby tending to increase the voltage on the emitter electrodes ofthe transistors TR151 to TR1S4, inclusive, and to bias the emitters more positive with respect to their base electrodes. Thereupon the current ow in each of the emitter-collector paths of the transistors is increased by an amount'in aggregate substantially equal to the 15 ma. current flow yin the conductor 197. Thus, the current ow through the load resistor 159 is substantially unaffected, and ythe potential on the conductor 197 and the terminal A207 is maintained at +50 volts, whereby the bias voltage furnished from the common terminal 207 to the other of 4'the line circuits 220A, etc., is maintained constant, and these line circuits are maintained in their normal operating state.

In response to the alarm signals of the temperature alarm 187 and the temperature alarm lamp LP227, the attendant at lthe central station 10takes vproper steps to remedy the high temperature condition existing at the temperature control station 20, and in addition, proceeds to disable the line circuit 220 until the high temperature condition thereat is remedied; whereupon the common equipment at the central station is restored and made available to register any trouble or temperature alarm conditions that may occur thereafter at any of the other of the temperature control stations 30, etc.

Specifically, in order to disable the line circuit 220, the line circuit control switch 240 associated with the line circuit 220 is operated from its normal position through its cut-off to its trouble-test'position. As the line circuit control switch 240 is moved through its cut-ott position, the circuit extended to the temperature-sensitive impedance device 250 at the temperature control station 20 v ia the line conductors 246A and 246B is interrupted at ontacts 242 and 244. 'At this time when thepreviously traced circuit through transistor TR222 is interrupted so' that TR222 is non-conductive, the base potential of TR222 is raised from +56 volts to -10 volts, the collector potential of TR221 and the emitter potential of TR223 is changed to approximately -60 volts so that TR221 and '1`R223 are biased conductive in the base-emitter paths thereof. Thereupon, transistors TR223 and TR221 are conductive in a path extending from +60 volts at terminal 204 through resistor 228, the collector-emitterpath of `TR223, resistor 232, the collector-emitter path of TR221 and resistor 234 to -60 volts at terminal 203; the transistor TR221 is non-conductive; and the temperature alarm lamp LP227 is extinguished. The current drawn through the conductor 196 of the temperature alarm controller 140 islreduced from 2 ma. to Zero so that the transistor TR141 thereof is rendered non-conductive and the relay R146 is restored. Thereupon, the previously traced circuit for operating the temperature alarm 187 is interrupted at contacts 146'. Incidental to the conduction of the transistor TR223, the trouble -alarm lamp LP230 is discharged and illuminated in a manner as described in detail hereinafter. However, the lamp LP230 merely flashes, for, as the line circuit control switch 240 passes through its cutoifposition into its trouble-test position, the contacts 241 and'243 'thereof are closed thereby completing a circuit for rendering the transistor TR223 and the trouble alarm lamp LP230 non-conductive.

Specifically, upon closing, the contacts 241 and 243 connect a'load resistor 247 to the line circuit 220. in the place of the circuit extended to the temperature-sensitive impedance device 250 over the line conductors 246A and 246B.Y .The impedance of the load resistor 247 is substantially equal to that of the load resistor 252 in the temperature-sensitive impedance device 250 so that the line circuit `220 is placed in condition for normal operation whereupon a 4 ma. current flows in a path from +60 volts on the terminal 204 through resistor 224, contacts ,243, resistor 247, contacts 241, resistors 231 and232,

the collector-emitter path of the transistor .TR221, and the resistor 234 to -60 volts on the terminal 203. The potential at the junction between resistors 231 and 232 changes from approximately -50 to 48 Volts so that the transistor T R223 is biased non-conductive in the -baseemitter path thereof and conduction through the collector-emitter path thereof ceases so that collector voltage thereof changes to +40 volts and the trouble alarm lamp LP230 is extinguished. The line circuit control switch 240 is retained in its trouble alarm test position until the high temperature condition in the temperature control station 20 is corrected so that the temperature-sensitive impedance device 2 50 will again exhibit a high impedance to current ow therethrough; at which time the line circuit control switch 240 is returned to its normal position and the circuit for operating` the line circuit 220 is completed through the temperature-sensitive impedance device 250. The line circuit 220 is, at this time, then in its normal operating state, and is prepared to respond to another high temperature condition that may occur at the temperature control station 20, or to an open circuit condition that may develop in the line conductors 246A and 246B.

Section 5. -v0peration of the high temperature alarm system when aj: open circuit condition vdevelops in any of the line 'co'niuctos Assuming now that the circuit, extended via the line conductors 246A and .246B through the temperaturesensitive impedance device 250 at the temperature control station 2 0for operating the line circuit 220 is interrupted by an open line condition in one of the conductors, such, for example, as the line conductor 246B, the normal 4 ma. current ow through the path including the line conductors, the resistors 231 and 232 and the collector-emitter path of the transistor TR221 is interrupted. Thereupon, the voltage at the emitter electrode of the transistor TR223is changed from approximately 48 volts4 to approximately -60 volts on the terminal 203. At this time the transistor TR223 is biased conductive in the baseemitter path thereof so that approximately 3.3 ma. ows from the emitter thereof of which approximately 1.3 ma. is drawn from the collector electrode through the resistor 228, the additional 2 ma. being Idrawn through the base electrode of TR223. Thus, the 3.3 ma. tlows from the emitter of TR223 through resistor 232, the collectoremitter path of TR221 and resistor 234 to +60 volts on the terminal 203, whereby the emitter electrode of TR223 is established at approximately -50 volts and the collector electrode is bottomed at approximately -50 volts. At this time then, the potential across the series connected resistor 229 and trouble lamp LP230 is increased from approximately l volts to 100 volts whereby the trouble lamp LP230 breaks down and sustains a 2 ma. current ow therethrough from the terminal 205, whereby the trouble lamp LP230 is illuminated to indicate an open line condition in the line 246. Thereupon, the collector current of TR223 is approximately 3 ma., with l ma. owing through resistor 228, and the base current is reduced from 2 ma. to approximately .3 ma. Thus, the transistors TR221 and TR223 and the trouble lamp LP230 are conductive, and the transistor TR 222 is held nonconductive by a reverse bias of approximately 3.5 volts between the base and the emitter electrode thereof. The 2 ma. current owing through the trouble lamp LP230 is drawn through the conductor 195 from the trouble alarm controller 130. In the trouble alarm controller 130, the 2 ma. current tlowing in the conductor 195 is drawn in part through the current limiting resistor 133 and in part through the diode D135 so that the voltage on the base electrode of the transistor TR131 is established at approximately +47 volts, whereby the transistor TR131 is biased conductive in the emitter-base path thereof and supports a current flow therethrough and current tlows via the emitter-collector path thereof and relay R136 from +50 volts at the junction between resistors 120 and 121 to +27.5 volts so that the collector electrode of TR131 is bottomed at approximately +50 volts. Due to the voltage drop through the base resistor 132 caused by the emitter-base path current the voltage on the terminal 205 is established at +48 volts which does not affect the operation of the trouble lamp LP230. The current ow through the emitter-collector path of TR131 energizes and operates the trouble relay R136, thereby closing at contacts 137 the previously traced circuit from +60 volts on the power conductor 194 via contacts 184 of the test switch 180 and the trouble alarm 190 to -60 volts on the power conductor 193. At this time the trouble arm 190 is operated thereby rendering an audible alarm to the attendant at the central station that an open line condition exists on one of the control lines 246, etc., of the high temperature alarm system. The attendant then checks all of the trouble alarm lamps in the system and is given notice by the illuminated condition of trouble alarm lamp LP230 that the open line condition exists in the control line 246.

Reconsidering the initial operation of the transistor TR223 in the line circuit 220, the 2 ma. current owing in the base-emitter path thereof is drawn through the conductor 197 from the voltage regulator 150. Therein, the 2 ma. current demand of conductor 197 tends to be supplied initially through the load resistor 159 from the power conductor 194, whereby the potential drop through the resistor 159 is increased and the voltage on the conductor 197 and the emitter electrodes of the transistors TR151 to TR154, inclusive, tends to change from -50 volts to approximately -51 volts. Thus, the voltage on the emitter electrodes tends to become negative with respect to the voltage on the base electrodes, thereby decreasing the current flow through the emitter-collector paths of the transistors TR151 to TR154, inclusive, by an aggregate of 2 ma. so that the current flow through the load resistor 159 remains substantially constant, and the voltage extended to the terminal 207 by the conductor 197 remains at -50 volts. Thereafter, when the current ow through the base of the transistor TR223 in the line circuit 220 is reduced to .3 ma. the emitter electrodes of the transistors TR151 to TR154 inclusive, in the voltage regulator 150 tends to swing positive with respect to the base electrodes thereof, whereby the total current flow through the transistor is increased by an amount of 1.7 ma. to compensate for the decrease in current drawn through the conductor 197.

Considering now the further operation of the line cir= cuit 220, in response to the audible alarm rendered by the 4operation of the trouble alarm 190, the attendant at the central station 10 operates the line circuit control switch 240 associated with the control line 246 from its normal position through'its cut-off and into its trouble-test position, whereby, as explained in Section 4 hereof, the line conductors 246A and 246B are disconnected from the line circuit 220 at contacts 242 and 244, and the load resistor 247 is connected to the line circuit 220 at contacts 241 and 243. A circuit is then completed from +60 volts at the terminal 204 via resistor 224, contacts 243, resistor 247, contacts 241, resistors 231 and 232, the collectoremitter path of the transistor TR221 and resistor 234 to +60 volts on the terminal 203. The current through the above traced path is increased to 4 ma. so that the transistor TR223 is biased non-conductive at the emitter-electrode thereof by the -48 volts established at the junction between resistors 231 and 232. 'Ihe collector potential thereupon increases from -50 volts to approximately +40 volts, whereby the potential across trouble lamp LP230 is reduced to below the required conduction potential thereof and the trouble alarm lamp LP230 is rendered non-conductive. The current drawn through the conductor 195 of the trouble alarm controller 130 is reduced from 2 ma. to zero so that the transistor TR131 therein in rendered non-conductive and the relay R136 is restored. Thereupon, the previously traced circuit for operating the trouble alarm 190 is interrupted at contacts 137. At this time, the line circuit 220 is disabled until the open line condition in control line 246 is repaired, and the common equipment at the central station 10 is restored to normal and again conditioned to respond to trouble alarm conditions and temperature alarm conditions that may occur in the other of the line circuits 220A, etc., of the high temperature alarm system. Thereafter, when the open line condition in the control line 246 is repaired the line circuit control switch 240 of the line circuit 220 is restored from its trouble-test position to its normal position, thereby completing the previously traced circuit through the temperature-sensitive impedance device 250 at the temperature control station 20 and rendering the line circuit 220 normally operative to detect any high temperature condition that may arise at the temperature control station 20 and any further interruptions that may occur in the conductors of control line 246.

Section 6.-0peraton of the high temperature alarm system when one of the line conductors therein s grounded Now assuming that one of the conductors in one of the control lines extending between the line circuits 220, etc., and the corresponding temperature control stations 20, etc., is accidentally grounded, the one of the line conductor ground lamps LP116 and LP117 in the grounded line detector unit corresponding to the polarity of the grounded conductor is illuminated, thereby rendering the attendant at the central station 10 an indication of the fault condition existing in the high temperature alarm system. Specilically, assuming that the line conductor 246A of the control line 246 extending from the line circuit 220 to the temperature control station 20 is accidentally grounded, the voltage on the conductor 246A is changed from -40 volts to zero volts, and the voltage on the line conductor 246B is changed from -40 volts to -80 13 volts. Thus, the potential on the negative power conductor 193 is established'at' -20volts andthe potential on the positive powercondiictor 194 is established at"- l00 volts. In'the grounded line ldetectoi- 110`,the change in voltage on the power conductors 193 `an`d-194 changes Vthe voltages at the junctions between the "resistorsf'of the voltage divider therein so that the-voltage at"the junction'between resistors 114 and 115 fis yraisedy from'+50-volts`to +90 volts, and the-potential across the electrodes of the negative line conductor ground-lamp LP117? is established at substantially 90v volts so that the lamp breaks-down and is illuminated.

In accordance with the illumination of the lamp LP117, the attendant at the central station is given notice that the negative conductor of one of fthe control lines in the system is grounded, andthereafter attempts to locate the one control line in which the grounded negative conductor condition exists by individually operating each of the line circuit control switches 240, etc., one at a time, from its normal positionthrough the cut-olf and into the troubletest position and then back to its normal position until the operatednegative conductor lampLP117 is extinguished. Specifically, when 'the line circuit lcontrol switch 240 is operated into its trouble-test position, the circuit extended Via the nconductors 246A and 246B is interrupted first at 4contactsV 242 and 244, and Iirnmediately 'thereafter the load resistorl247 is connected at contacts 241'a`nd 243 to the line circuit 220. At thisI time then, the grounded line conductor 246A is disconnected from the line circuit 220 and the line circuit 2,20 is normally operating so that the voltage on-the powerconductor 193 is changed from -V2O`volts to -60 volts, and the voltage on the power conductor `194 is changed from +100 volts to +60 volts. In accordance with the shift in the voltages on the power conductors 193 and 194, the voltages at the junction between resistors 113, 114 and 115 are shifted accordingly so thatthe potential across 'the electrodes of both line conductor ground lamps LPI-16 and LP117 is substantially below therequired conduction potential thereof; hence, Ltheilluminating current flow through the lamp LP117 is interrupted and the lamp isl extinguished. `At this time the line circuit 220 is disabled and the common equipment at the vcentral station 10 is conditioned to respondto temperature alarm conditions or trouble alarm conditions that may occur in anyl other of the line circuits 2420A, etc., or to a grounded conductor condition that may occur in any other of the control lines 248, etc. i

The attendant at the central station 10 retains the line circuit control switch 240 in its trouble-test position until the grounded negative line conductor condition of the control line 246 is repaired, whereupon the line circuit control switch 240 is returned to its normal position and the circuit extended via the line conductors 246A and 246B through the temperature-sensitive irnpedance device 250 is again connected to theline circuit 220 so that the line circuit 220 is in its normal operating state with the transistor TR221 conducting. At this time, each of the line circuits 220, etc., are prepared to respond to a high temperature condition, an open line condition, and a grounded line conductor condition. l

Now assuming that the line conductor 246B in the circuit extending from the line circuit 220 to the temperature control station 20 is accidentally grounded, the voltage on the conductor 246B is 'changed lfrom +40 volts to zero volts and the voltage on the line conductor 246A is changed from-40 volts'to S0 volts. Accordingly, the voltage on the negative power conductor 193 is changed to --100 volts, and the voltage on the positive power conductor 194 is Vchanged to +20 volts and the voltages atfthe junctions between resistors 113, 114 and 115 in the grounded line detector 110 are shifted so that the potential across the electrodes of the positive line conductor ground'lamp LP116 is approximately 90 fY tthat the positive line"conductorofljoneofthe control 11,1;

-|246,` eticf," the 'high temperature alarm system" is grounded. 'i t` `ln accordance with the illuminationof the lamp LP116',

theatte'ndantat the central station 410 atteinjnts to locate fthefoiie control line iniwhich thejgrounded positive conductorl `condition exists byindividually operatingfeachrof the line circuit switches 240,:etc., Vone at time', vfrom its nor- 'it'ion and then back to its normallposition until the erated positive conductor lamp4 H2116 is extinguished.

,'Specically, whenithe lline circuit control switch 240 is operated into its trouble-test` position, lthe circuit extended viaA the vconductorsiZrttSA and 246B is interrupted first at ycontacts "242 and y244, and immediately: thereafter the la'd resistor 2147 is connectedatcontacts 241 and 243 to the line circuit 220. lAtthis time'then, the'grounded line conductor 246B is disconnected from the line circuit 220 and the line circuit 220 is normally operating, so that the voltage on the power conductor 193 is vchanged from "l00fv'olts to 60 volts, and the voltage onthe power conductor 194 is changed from +20 volts to +60 volts. `lri'a'ccordance with the shift'in theI voltages on the power conductors 193and 194, the voltages at the junctions be- 'tween yresistors .113, 114`and 1,15 are shifted accordingly "soithat'the potential across the electrodes of both line conductor ground'lam'ps vLPlltS and LP117 is substantial- =ly'bel'ow the conduction potential thereof; hence, the illuminating currentowhrough the lamp LP116 is in- `t`erri1pted, the lamp 'is extinguished, the` line circuit 220 is disabled, andjas'p'reviou'sly explained, the high temperaturealarnf system is conditioned to respond to alarm and rfault-co`nditionsr Ythat may occur 4in `the other ofthe 'line vcircuits and control conductors. The attendant at the central station 10 retains the line'circuit control switch 240 in its trouble-test position until the`` grounded positive line conductor'condition of the control l1ine'246-is repaired, whereupon the line circuit `control switch`240 is returned toits normal position and the fcircuit'extended via the line conductors'246A and *246B through the temperature-sensitive impedance device 250'is again connected ltothey line ,220', so that vthe line circuit 220 is in its normal operating state with the transistor TR221 conducting. :this time, each of the line' circuits 220, etc., are prepared to respond to a high temperature condition, anV open line condition, and a Agrounded line conductor condition.

Section 7.--Testz'ng the operation of the high temperature i alarm system The high temperature alarm system includes arrangements so thatV each of the line circuits may be tested for temperature alarm operation and trouble alarm operation easily and quickly from the attendants position at the central station `10.

Assuming first that it is desired to test the operation of the line circuit 220 for a high temperature alarm condition, the test switch 180 isl operated from its normal position into itsitemperature alarm test position, thereby closing contacts l181 and182 and opening contacts 183 thereof. At contacts 181, the previously mentioned circuit for applying 60 volts from thepower conductor 193 to the emitterV electrodes of the transistors TR161 and TR1V62 of the flasher unit is closed, whereby the flasher unit is operated.

Specifically, when contacts 181 are closed, one of the transistors, for example, TR161, is rendered conductive and the other TR16Z, is rendered non-conductivedue to the lack of positive identity between the operating char- :acteristics of the transistors. Thereupon, current flows Vin the circuit from `+60 volts via resistor 167, the capacitor C171 Vand the base-emitter path of TR161 to -60v01t5 and conduction in TR161 continues until the capacitor C171 charges and substantially blocks the current flow to the base thereof. With TR161 conducting the collector thereof is at substantially -60 volts and a large current flows from +60 volts via resistors 164 and 165 and the collector-emitter path of TR161 to -60 volts so that the junction between resistors 164 and 165 is at substantially volts and current flows thereto from +60 volts via resistors 167 and 168, the collector-base path of TR162 and capacitor C172, thereby biasing the transistor TR162 non-conductive and establishing the collector voltage thereof at slightly less than +60 volts. Thereafter when the capacitor C171 is charged, the transistor TR161 is rendered non-conductive in the base-emitter path so that the collector current is cut olf and the collector thereof and the junction between resistors 164 and 165 is established at approximately +60 volts. Thereupon current ows from +60 volts on the power conductor 194 via the resistor 164 and capacitor C172 through the base-emitter path of TR162 to -60 volts at the emitter thereof; whereby the latter transistor is rendered conductive and the collector voltage thereof is established at approximately -60 volts and the voltage at the junction between resistors 167 and 168 is established at approximately 0 volts so that a small reverse current is drawn through the collector-base path of transistor TR161 and capacitor C171 in order to discharge the latter capacitor. Thereafter, when the capacitor C172 is charged sufficiently to substantially cut otf the current in the base-emitter path of TR162, the latter transistor is rendered non-conductive so that the collector voltage thereof is established at approximately +60 volts and the transistor TR161 is again rendered conductive. From the above description it is seen the voltage at the collector of TR162 changes abruptly from approximately +60 volts to approximately -60 volts and back to +60 volts to establish a substantially square wave voltage signal thereon. This voltage is employed to alternately illuminate and extinguish the test position lamp LP171 which has its anode electrode connected to approximately +50 volts at the junction between resistors 169 and 17S and its cathode electrode connected to the collector of TR162. When the potential on the collector of TR162 is at approximately +60 volts the lamp LP171 is extinguished and when the potential on the collector of TR162 is at approximately -60 volts the lamp LP171 conducts current and is illuminated. In this manner the test lamp LP171 is ashed at a rate of about five times a minute to indicate to the attendant at the central station that the test switch 180 is in one of its test positions. As previously pointed out, the closing of contacts 182 and the opening of contacts 183 substitutes the series connected resistor 188 and the temperature alarm lamp LP189 for the temperature alarm 187 in the previously traced operating circuit therefor.

Thereafter, the attendant operates the line circuit control switch 240 from its normal position to its temperature alarm test position. In the temperature alarm test position a circuit is closed vat contacts 245 for effectively shortcircuiting the load resistor 252 of the temperature-sensitive impedance device 250 so that a low impedance condition is presented to the line circuit 220 and the line circuit 220 is operated as though a high temperature condition exists at the temperature control station 20, and in the manner as described in Section 4 hereof so that the transistor TR221 is substantially non-conducting and the transistor TR222 is conducting. Responsive to the operation of the transistor TR222, the temperature alarm lamp LP227 is illuminated and the temperature alarm controller 140 is operated so that relay R146 operates to close at contacts 147 the circuit for illuminating the temperature alarm test lamp LP189. In this manner the attendant at the central station 10 is rendered notice that the line circuit 220 operates properly in response to high temperature alarm conditions; thereuponthe line crcuitcontrol switch 240 is operated from its temperature alarm test ble alarm test lamp LP192 to again flash, and finally confzs position through its normal position to its cut-ott position. As the switch 240 progresses from its normal position to its cut-off position the operating circuit through the line circuit 220 is interrupted at contacts 242 and 244 so that the transistor TR222 is rendered non-conductive, the alarm lamp LP227 is extinguished, the relay R146 in the temperature alarm controller is restored, and the operating circuit through the temperature alarm test lamp LP189 is interrupted so that the lamp LP189 is extinguished; whereupon the line circuit control switch 240 is restored to its normal position thereby recompleting the circuit over the line conductors 246A and 246B from the line circuit 220 through the temperature-sensitive control device 250. At this time, the line circuit 220 is in its normal operating state and may be operated in response to either high temperature condition, an interrupted line conductor condition or a grounded line conductor condition to render an alarm at the central station 10. At thc completion of the temperature alarm test, the test switch 180 is operated from its temperature alarm test position to its normal position, so that the flasher unit is rendered inoperative and the temperature alarm 187 is substituted for the temperature alarm test lamp 189 in the operating circuit therefor. Thus, at this time, all of the equipment in the high temperature alarm system is rendered normally operative and responsive to high temperature conditions, interrupted line conductor conditions, and grounded line conductor conditions that may occur therein.

Now, assuming that it is desired to test the operation of the line circuit 220 for a trouble Ialarm condition, the test switch is operated from its normal position into its trouble alarm test position, thereby closing contacts and 186 and opening contacts 184 thereof. At contacts 186, the previously mentioned circuit for applying -60 volts from the power conductor 193 to the emitter electrodes of the transistors TR161 and TR162 of the asher unit is operated in a manner as explained in this section to ilash the test position lamp LP171 at the rate of five times a minute in order to indicate to the attendant at the central station 10 that the test switch 180 is in one of is test positions. As previously pointed out, the closing of contacts 185 and the opening of contacts 184 substitutes the series connected resistor 191 and the trouble alarm test lamp LP192 of the trouble alarm 190 in the previously traced operating circuit therefor.

Thereafter the attendant operates the line circuit test switch 240 from its normal position through its cut-off position and into its trouble-test position, whereby the circuit extended via line conductors 246A and 246B to the temperature control station 20 is interrupted at contacts 242 and 244, presenting for a brief interval an interrupted line conductor condition to the line circuit 220, and whereby thereafter the load resistor 247 is connected to `the line circuit 220 at contacts 241 and 243 in order to render the line circuit normally operating. During the brief interval that the interrupted line conductor condition is presented to the line circuit 220, the transistor TR223 is operated in the manner as described in Section 5 hereof so that the trouble alarm lamp LP230 is flashed and the relay R136 in the trouble alarm controller 130 is operated and restored to close and interrupt 'at contacts 137 the previously prepared circuit through the trouble alarm test lamp LP192 so that the lamp LP192 is flashed.

Thus, when the trouble alarm lamp LP230 and the trouble' alarm test lamp LP192 are flashed to the `attendant at the central station 10 is rendered notice that the line circuit 220 operates properly in response to an interrupted line conductor condition. Thereupon, the line circuit control switch 240 is operated from the trouble-test position through the cut-oit position and into the normal position causing the trouble alarm lamp LP230 and the trounecting the line conductors 246A and 246B to the line circuit 220, whereby the line circuit 220 is rendered normally operating and is prepared to respond, in the manner as previously explained, to a high temperature condition, an interrupted line conductor condition and a grounded line conductor condition. Further, the test switch 180 is operated from its trouble alarm test position to its normal position thereby interrupting at contacts 186 the circuit for operating the flasher 160 and opening contacts 185 and closing contacts 184 to replace the trouble alarm test lamp LP192 with the trouble alarm 19@ in the previously traced circuit therefor. At this time the high temperature alarm system is in its normal operating state and is prepared to respond to a high temperature condition that may occur at any of the temperature control stations 20, etc., thereof and is prepared to respond to an interrupted line conductor condition and a grounded line conductor condition that may occur in any of the control lines 246, etc.

The alarm system of the invention has been described in terms of a high temperature alarm system, and while at present this is considered to be the preferred embodiment of the invention it is understood that the alarm system might easily be adapted to a low temperature alarm system, and is not limited to rendering temperature alarms but may be adapted to respond to any one of a number of other physical phenomenon including, for example, light, pressure, and humidity. In any event, it is clear that various modifications may be made in the described embodiment, and it is intended to cover in the appended claims all such modifications that fall within the true spirit and scope of the invention.

What is claimed is:

1. An alarm system comprising a plurality of control stations; a central station; a plurality of two-conductor lines respectively extending from said control stations to said central station; each of said control stations being provided with a device selectively responsive to normal and abnormal conditions thereat respectively to establish corresponding normal and abnormal impedance connections between the conductors of the line extending thereto; a source of power provided with a reference voltage terminal and a low voltage terminal and a high voltage terminal; a plurality of line circuits at said central station and respectively associated with said lines; each of said line circuits including a first connection from said high voltage terminal to one of the conductors of the associated line, a first biasing resistor, a second biasing resistor, a third biasing resistor, a second connection from said high voltage terminal via said first biasing resistor and said second biasing resistor and said third biasing resistor to said reference voltage terminal, a first control resistor, a second control resistor, a first transistor provided with a base connected to the junction between said second biasing resistor and said third biasing resistor and a collector connected via said first control resistor to the other of the conductors of the associated line and an emitter connected via said second control resistor to said reference voltage terminal, and a second transistor provided with a base connected to the junction between said first control resistor and the collector of said first transistor and a collector connected to the junction between said first biasing resistor and said second biasing resistor and an emitter connected to said low voltage terminal, whereby in any one of said line circuits the first transistor is rendered conductive in response to a normal impedance connection at the corresponding control station and the second transistor is rendered conductive in response to an abnormal impedance connection at the corresponding control station and wherein the first transistor is rendered substantially non-conductive in `response to conduction of the second transistor so that the second transistor is maintained conductive irrespective of subsequent change of the impedance connection at the corresponding control station; an alarm indicator at said central station; and means responsive to conduction of the second transistor in any one of said line circuits for selectively operating said alarm indicator to signify the abnormal condition at the corresponding one of said control stations.

2. An alarm system comprising a plurality of control stations; a central Station; a plurality of two-conductor lines respectively extending from said control stations to said central station; each of said control stations being provided with a device selectively responsive to normal and abnormal conditions thereat respectively to establish corresponding normal and abnormal impedance connections between the conductors of the line extending thereto; a source of power provided with a reference voltage terminal and a low voltage terminal and a high voltage terminal; a plurality of line circuits at said central station and respectively associated with said lines; each of said line circuits including a first connection from said high voltage terminal to one of the conductors of the associated line, a iirst biasing resistor, a second biasing resistor, a third biasing resistor, a second connection from said high voltage terminal via said first biasing resistor and said second biasing resistor and said third biasing resistor to said reference voltage terminal, a first control resistor, a second control resistor, a third control resistor, a first transistor provided with a base connected to the junction between said second biasing resistor and said third biasing resistor and a collector connected via said first control resistor and said second control resistor to the other of the conductors of the associated line and an emitter connected via said third control resistor to said reference voltage terminal, a second transistor provided with a ibase connected to the junction between said first control resistor and the collector of said rst transistor and a collector connected to the junction between said rst biasing resistor and said second biasing resistor and an emitter connected to said low voltage terminal, a fourth biasing resistor, and a third transistor provided with a base connected to said low voltage terminal and a collector connected via said fourth biasing resistor to said high voltage terminal and an emitter connected to the junction between said first control resistor and said second control resistor, whereby in any one of said line circuits the first transistor normally is conductive in response to a normal impedance connection extended thereto from the corresponding control station and the second transistor is conductive in response to an abnormal impedance connection extended thereto from the corresponding control station and the first and third transistors are conductive in response to an interruption in the impedance connection normally extended thereto from the corresponding control station and wherein the rst transistor is substantially non-'conductive in response to conduction of the second transistor so that the second transistor is maintained conductive irrespective of subsequent change at the corresponding control station of the abnormal impedance connection to the normal impedance connection; a first alarm indicator at said central station; means responsive to the conduction of the second transistor in any one of said line circuits for selectively operating said first alarm indicator to signify the abnormal condition at the corresponding one of said control stations; a second alarm indicator at said central station; and means responsive to the conduction of the third transistor in any one of said line circuits for selectively operating said second alarm indicator to signify an interruption in the connection between said one line circuit and the corresponding control station.

3. An alarm system comprising a plurality of control stations; a central station; a plurality of two-conductor lines respectively extending from said control stations to said central station; each of said control stations being provided with a device selectively responsive to normal and abnormal conditions thereat respectively to establish corresponding normal and abnormal impedance connections between the conductors of the line extending thereto; a source of power provided with a reference voltage terminal and a low voltage terminal and a high voltage terminal; a plurality of line circuits at said central station and respectively associated with said lines; each of said line circuits including a first connection from said high voltage terminal to one of the conductors of the associated line, a first biasing resistor, a second biasing resistor, a third biasing resistor, a second connection from said high voltage terminal via said first biasing resistor and said second biasing resistor and said third biasing resistor to said reference voltage terminal, a first control resistor, a second control resistor, a first transistor provided with a base connected to the junction between said second biasing resistor and said third biasing resistor and a collector connected via said first control resistor to the other of the conductors of the associated line and an emitter connected via said second control resistor to said reference voltage terminal, a second transistor provided with a base connected to the junction between said first control resistor and the collector of said first transistor and a collector of said first transistor and a collector connected to the junction between said first biasing resistor and said second biasing resistor and an emitter connected to said low voltage terminal, and an output terminal connected to the junction between said first and second biasing resistors and the collector of said second transistor, whereby in any one of said line circuits the first transistor normally is conductive in response to a normal impedance connection at the corresponding control station and the second transistor is conductive in response to an abnormal impedance connection at the corresponding control station and wherein the output terminal has a marking Voltage applied thereto in response to conduction of the second transistor and wherein the first transistor is substantially non-conductive in response to conduction of the second transistor so that the second transistor is maintained conductive irrespective of subsequent change of the impedance connection at the Corresponding control station; and indicator means at said central station comprising a load resistor and a plurality of diodes of the gas discharge type respectively associated with said line circuits and each provided with a cathode connected to the output terminal of the corresponding line circuit and an anode connected via said load resistor to said high voltage terminal, whereby any one of said diodes is discharged and illuminated in response to said marking voltage on the output terminal of the corresponding line circuit; an alarm device at said central station; and means responsive to the illumination of any one of said diodes for operating said alarm device to signify the abnormal condition at the one of the control stations corresponding to the illuminated diode.

4. An alarm system comprising a plurality of control stations; a central station; a plurality of two-conductor lines respectively extending from said control stations to said central station; each of said control stations being provided with a device selectively responsive to normal and abnormal conditions thereat respectively to establish corresponding normal and abnormal impedance connections between the conductors of the line extending thereto; a source of power provided with a reference voltage terminal and a low voltage terminal and a high voltage terminal; a plurality of line circuits at said central station and respectively associated with said lines; each of said line circuits including a first connection from said high voltage terminal to one of the conductors of the associated line, a first biasing resistor, a second biasing resistor, a third biasing resistor, a second connection from said high voltage terminal via said first biasing resistor and second biasing resistor and said third biasing resistor to said reference voltage terminal, a first control resistor, a second control resistor, a third control resistor, a first transistor provided with a base connected to the junction between said second biasing resistor and said third biasing resistor and a collector connected via said first control resistor and said second control resistor to the other of the conductors of the associated line and an emitter connected via said third control resistor to said reference voltage terminal, a second transistor provided with a base connected to the junction between said first control resistor and the collector of said first transistor and a collector connected to the junction between said first biasing resistor and said second biasing resistor and an emitter connected to said low voltage terminal, a fourth biasing resistor, a third transistor provided with a base connected to said low voltage terminal and a collector connected via said fourth biasing resistor to said high voltage terminal and an emitter connected to the junction between said first control resistor and said second control resistor, a first output terminal connected to the junction between said first and second biasing resistors and the collector of said second transistor, and a second output terminal connected to the junction between said fourth biasing resistor and the collector of said third transistor, whereby in any one of said line circuits the first transistor' normally is conductive in response to a normal impedance connection extended thereto from the corresponding control station and the second transistor is conductive in response to an abnormal impedance connection extended thereto from the corresponding control station and the first and third transistors are conductive in response to an interruption in the impedance connection normally extended thereto from the corresponding control station and wherein the first transistor is substantially non-conductive in response to conduction of the second transistor so that the second transistor is maintained conductive irrespective of subsequent change at the corresponding control station of the abnormal impedance connection to the normal impedance connection and wherein the first output terminal has a marking voltage applied thereto in response to conduction of the second transistor and wherein the second output terminal has the marking voltage applied thereto in response to conduction of' the third transistor; first indicator means at said central station comprising a first load resistor and a plurality of first diodes of the gas discharge type respectively associated With said line circuits and each provided with a cathode connected to the first output terminal of the corresponding line circuit and an anode connected via said first load resistor to said high voltage terminal, whereby any one of said first diodes is discharged and illuminated in response to said marking voltage on the first output terminal of the corresponding line circuit; a first alarm in said central station; and means responsive to the illumination of any one of said first diodes for operating said first alarm to signify the abnormal condition at the one of the control stations corresponding to the illuminated first diode; second indicator means at said central station comprising a second load resistor and a plurality of second diodes of the gas discharge type respectively associated with said line circuits and each provided with a cathode connected to the second output terminal of the corresponding line circuit and an anode connected via said second load resistor to said high voltage terminal, whereby any one of said second diodes is discharged and illuminated in response to said marking voltage on the second output terminal of the corresponding line circuit; a second alarm at said central station; and means responsive to the illumination of any one of said second diodes for operating said second alarm to signify the interruption of the impedance connection normally extended from the one of the control stations to the illuminated second diode.

5. An alarm system comprising a plurality of control stations; a central station; a plurality of two-conductor lines respectively extending from said control stations to said central station; each of said control stations being provided with a device selectively responsive to normal and abnormal conditions thereat respectively to establish corresponding normal -and .abnormal impedance :connections between the conductors of the line-extending thereto; a source of powerprovided-with a reference voltage terminal and a low voltage terminal and an high voltage terminal; a plurality of line circuits at said central station and respectively associated With-said lines; each of said line circuits including a pair of input terminals, a first connection from said high voltage terminal to one or" said input terminals, a rst biasing resistor, a second biasing resistor, a third biasing resistor, a second connection from said high voltage terminal via said rst biasing resistor andsaid second biasing resistor and said third biasing resistor to said vreference voltage terminal, a first control resistor, a -second control resistor, a third control resistor, a first transistor .provided with a base connected to the junction between -said second biasing resistor and said third biasing .resistor and a col-lector connected via said first control resistor and said second control resistor to the other of said1input terminals and an emitter connected via said third control resistor to said reference voltage terminal, a second transistor provided with a base connected to the junction between said first control resistor and the collector of said first transistor and a collector connected to thejunction-between said first biasing resistor and said second biasing resistor and an emitter connected to said low voltage terminal, afourth biasin 7 resistor, a third transistor provided with a base connected to said low voltage terminal and a collector connected via vsaid fourth biasing resistor to said high voltage terminal and an emitter connected to the junction between said first control resistor and said second control resistor, a first outputterminal connected tothe junction between said first and second biasing resistors and the collector-.of said second transistor,.and va secondoutput terminal connected to .thejunction-between `said fourth biasing resistor and the collectorof said third transistor; a-plurality of line .circuit control means respectively disposed between the Vline 4.circuits .andthe corresponding lines; each of said line Vcircuit control means including an impedance devicehaving .an .impedance value .corresponding to that of the normal Aimpedance .connection in the corresponding'control station and a control switch selectively operable into .a first position and a second position and `a third position, thecontrol switch of any one of said line .circuit control means `being normally in the first position and completing .thereatconnections from the conductors of the corresponding line respectively to thefinput terminals of the corresponding line circuit, whereby inany one of said line circuitsthe firsttransistornormally islconductive inresponse to .a normal impedance connection extended theretofrom the corresponding control stationand the second transistor. is conductive in response to an abnormal impedance .connection extended thereto from the. corresponding control station and the first and third transistors are .conductivein response toan interruption.` in the impedance connection normally extended thereto from the .corresponding controlstation and wherein the first transistoris .substantiallynon-conductive inresponse to conduction of the secondtransistor so that the second transistor .is maintained conductive irrespective of subsequent changeatthe corresponding control station of the abnormal .impedancerconnection to the normal impedance connection and wherein the first output terminal has a marking voltage applied thereto in response to conduction of the secondtransistor and wherein Vthe secondoutput Iterminal has the marking voltage applied thereto in response to .conduction `of the third transistor, thecontrol switch of any oneof said line circuit control means:in ,the 4second position interrupting thereattheconnections between .the corresponding.v line circuit4 and the Yconductors of rthe corresponding linein order to. effect interrupted .impedance connection operation of the llineocircuit'sothat,the-first iand third transistors .thereof are conductive, thescontrtol switch of anyone of..said line;.circuittcontrolfmeansiin.the third position completing thereat connections between ytheimpedance device thereof and the corresponding 'line circuit in order to eiect normal impedance connection operation of the line circuit to that the first transistor thereof is conductive; irst indicator means at said central station comprising a first load resistor and a plurality of rst diodes of the gas discharge type respectively associated with said line circuits and each provided with a cathode connected to the first output terminal of the corresponding line circuit and an anode connected via said first load resistor to said high Voltage terminal, whereby any one of said first diodes is discharged and illuminated in response to said marking voltage on the first output terminal of the corresponding line circuit; a first alarm at said central station; means responsive to the illumination of any one of said first diodes for operating said first alarm to normally signify the abnormal condition at the one of the control stations corresponding to said illuminated first diode; second indicator means at said central station comprising a second load resistor and a plurality of second diodes of the gas discharge type respectively associated with said line circuits and each provided with a cathode connected to the second output terminal of the corresponding line circuit and an anode connected via said second load resistor to said high voltage terminal, whereby any one of said second diodes is discharged and illuminated in response to said marking voltage on the second output terminal of the corresponding line circuit; a second alarm at said central station; and means responsive to the illumination of any one of said second diodes for operating said second alarm to signify the interruption of the impedance connection normally extended from the one of the control stations corresponding to said illuminated second diode; whereby upon occurrence of an abnormal condition at any one ot' said control stations and pending the reinstatement of the normal condition thereat the operated first alarm is restored and the illuminated one of said first diodes is extinguished by operating the control switch of the corresponding line circuit control means from the first position through the second position and into the third position in order to operate the corresponding line circuit so that the second transistor is rendered non-conductive and the first transistor is rendered conductive; and whereby upon interruption of any one of the impedance connections normally extended between the control stations and the line circuits and pending the repair or" the interruption therein the operated second alarm is restored and the illuminated one of said second diodes is extinguished by operating the control switch of the corresponding line circuit control means from the first position into the third position in order to operate the corresponding line circuit so that the third transistor is rendered non-conductive.

6. An alarm system comprising a plurality of control stations; a central station; a plurality of two-conductor lines respectively extending from said control stations to said central stations; each of said control stations being provided with a device selectively responsive to normal and abnormal conditions thereat respectively to establish corresponding normal and abnormal impedance connections between the conductors of the line extending thereto; a source of power provided with a reference voltage terminal and a low voltage terminal and a high voltage terminal; a plurality of line circuits at said central station and respectively associated with said lines; each of said line circuits including a pair of input terminals, a first connection from said high voltage terminal to one of said input terminals, a first biasing resistor, a secondpbiasing resistor, a third biasing resistor, a second connection from said high voltage terminal viasaid first biasing resistor and said second biasing resistor and said third biasing resistor to said reference voltage terminal, a `first control resistor, a second control resistor, a third control resistor, a first transistorl provided with .a base connected tothe junction between said second biasing resistor and said third biasing resistor and a collector connected via said first control resistor and said second control resistor to the other of said input terminals and an emitter connected via said third control resistor to said reference voltage terminal, a second transistor provided with a base connected to the junction between said first control resistor and the collector of said first transistor and a collector connected to the junction between said first biasing resistor and said second biasing resistor and an emitter connected to said low voltage terminal, a fourth biasing resistor, a third transistor provided with a base connected to said low voltage terminal and a collector connected via said fourth biasing rcsistor to said high voltage terminal and an emitter connected to the junction between said first control resistor and said second control resistor, a first output terminal connected to the junction between said first and second biasing resistors and the collector of said second transistor, and a second output terminal connected to the junction between said fourth biasing resistor and the collector of said third transistor; a plurality of line circuit control means respectively disposed between the line circuits and the corresponding lines; each of said line circuit control means including an impedance device having an impedance value corresponding to that of the abnormal impedance connection in the corresponding control station and a test switch selectively operable into a first position and a second posiiton and a third positon, the test switch of any one of said line circuit control means being normally in the first position and completing thereat connections from the conductors of the corresponding line respectively to the input terminals of the corresponding line circuit, whereby in any one of said line circuits the first transistor normally is conductive in response to a normal impedance connection extended thereto from the corresponding control station and the second transistor is conductive in response to an abnormal impedance connection eX- tended thereto from the corresponding control station and the first and third transistors are conductive in response to an interruption in the impedance connection normally extended thereto from the corresponding control station and wherein the rst transistor is substantially non-conductive in response to conduction of the second transistor so that the second transistor is maintained conductive irrespective of subsequent change at the corresponding control station of the abnormal impedance connection to the normal impedance connection and wherein the first output terminal has a marking voltage applied thereto in response to conduction of the second transistor and wherein the second output terminal has the marking voltage applied thereto in response to conduction of the third transistor, the test switch of any one of said line circuit control means in the second position interrupting thereat the connections between the corresponding line circuit and the conductors of the corresponding line in order to effect interrupted impedance connection operation of the corresponding line circuit so that the first and third transistors thereof are conductive, the test switch of any one of said line circuit control means in the third position cornpleting thereat connections between the impedance device thereof and the corresponding line circuit in order to effect abnormal impedance connection operation of the corresponding line circuit so that the second transistor thereof is conductive; first indicator means at said central station comprising a first load resistor and a plurality of first diodes of the gas discharge type respectively associated with said line circuits and each provided with a cathode connected to the first output terminal of the corresponding line circuit and an anode connected Via said first load resistor to said high voltage terminal, whereby any one of said first diodes is discharged and illuminated in response to said marking voltage on the first output terminal of the corresponding line circuit; a first alarm at said central station; means responsive to the illumination of any one of said'rst diodes for operating said first alarm to normally signify the abnormal condition at the one of the control stations corresponding to said illuminated first diode; second indicator means at said central station comprising a second load resistor and a plurality of second diodes of the gas discharge type respectively associated with said line circuits and each provided with a cathode connected to the second output terminal of the corresponding line circuit and an anode connected via said second load resistor to said high voltage terminal, whereby any one of said second diodes is discharged and illuminated in response to said marking voltage on the second output terminal of the corresponding line circuit; a second alarm at said central station; and means responsive to the illumination of any one of said second diodes for operating said second alarm to signify the interruption of the impedance connection normally extended from the one of the control stations corresponding to said illuminated second diode; whereby any one of said line circuits may be tested for interrupted impedance connection operation by operating the test switch of the corresponding line circuit control means from the first position through the second position to the first position in order to render conductive and non-conductive the third transistor thereof and thereby to operate and restore the corresponding second diode and the second alarm; and whereby any one of said line circuits may be tested for abnormal impedance connection operation by operating the test switch of the corresponding line circuit control means from the first position through the third position and the second position to the first position in order to render conductive and non-conductive the second transistor thereof and thereby to operate and restore the corresponding first diode and the rst alarm.

7. In an alarm system including a control station, a central station, a two-conductive line extending from the control station to the central station, the control station being provided with a device selectively responsive to normal and abnormal conditions thereat respectively t0 establish corresponding normal and abnormal impedance connections between the conductors of the line extending thereto, and a line circuit at said central station associated with said two-conductor line; the line circuit combination comprising a source of power provided with a reference voltage terminal and a low voltage terminal and a high voltage terminal, a first connection from said high voltage terminal to one of the conductors of the associated line, a first biasing resistor, a second biasing resistor, a third biasing resistor, a second connection from said high voltage terminal via said first biasing resistor and said second biasing resistor and said third biasing resistor to said reference voltage terminal, a first control resistor, a second control resistor, a first transistor provided with a base connected to the junction between said second biasing resistor and said third biasing resistor and a collector connected via said first control resistor to the other of the conductors of the associated line and an emitter connected via said second control resistor to said reference voltage terminal, a second transistor provided with a base connected to lthe junction between said first control resistor and the collector of said first transistor and a collector connected to the junction between said first biasing resistor and said second biasing resistor and an emitter connected to said low voltage terminal, a third control resistor, an alarm indicator, and a third connection from the junction between said first and second biasing resistors and the collector of said second transistor via said third control resistor and said alarm indicator to said high voltage terminal, whereby in any one of said line circuits the first -transistor is rendered conductive in response to a normal impedance connection at the corresponding control station and the second transistor is rendered conductive in respose to an abnormal impedance connection at the corresponding control station and wherein the first transistor is rendered substantially non-conductive in response to'conduction of the second transistor so that the second transistor is ,maintained conductive irrespective of. subsequent change of the impedance con nection at the correspondingrcontnol station andA wherein the alarm indicator is operated in response to conduction of the second :transistor to signify ,the abnormal condition at the corresponding control station.

8. The alarm system including the line vcircuit combination set forth in claim 7, wherein said alarm .indicator is of the gas diode discharge type.

9. In an alarm system including a control station, a central station, and a two-conductor line extending from the control station to the central station, the control station being` provided with a device selectively responsive to normal and abnormal conditions thereat respectively to establish corresponding normal and abnormal impedance connections between the conductors of the line extending thereto, and a line circuit at the central station associated with the two-conductor line; the line circuit combination comprising a source of power provided with -a reference voltage terminal and a low voltage terminal and a high voltage terminal, a iirst 4connection from said high voltage terminal t-o a iirst of the conductors of the associated line, a rst biasing resistor, a second biasing resistor, a third biasing resistor, a second connection from said high voltage terminal via said rst biasing resistor and said second biasing resistor and said third biasing resistor to said reference voltage terminal, a irst control resistor, a second control resistor, a third control resistor, a first transistor provided with a base connected to the junction between said second biasing resistor and said third biasing resistor and a collector connected via said first control resistor .and said second control resistor to the .other of the conductors of the associated line and an emitter connected via said third control resistor to said reference voltage terminal, a second transistor provided with a base connected to the junction between said first control resistor and the collector of said iirst transistor and a collector connected to the junction between said rst biasing resistor and said second biasing resistor and Jan emitter connected to said low voltage terminal, a fourth biasing resistor, a third transistor provided with a base connected to said low voltage terminal and a collector connected via said fourth biasing resistor to said high voltage terminal and an emitter connected to :the junction between said rst control resistor and said second control resistor, a fourth control resistor, a first alarm indicator, a third connection from the junction between said rst and second biasing resistors and the collector of said second transistor via said fourth control resistor and said iirst alarm indicator to said high voltage terminal, a lifth control resistor, a second alarm indicator and a fourth connection from the junction between said fourth biasing resistor and the collector of said third transistor via said fifth control resistor and said second alarm indicator to said high voltage terminal whereby in any `one of said line circuits the rst transistor normally is conductive in response to a normal impedance connection extended thereto from the corresponding control station and the second transistor is conductive in response to an abnormal impedance connection extended thereto from the corresponding control station and the rst and third transistors are conductive in response to an interruption in Ithe impedance connection normally extended thereto from the corresponding control station and wherein the irst transistor is substantially non-conductive in response to conduction of the second transistor so that the second transistor is maintained conductive irrespective of subsequent change of the abnormal impedance connection to the normal impedance connection at the corresponding control station and the tirst alarm indicator is operated in response to conduction of the second transistor and the second alarm indicator is operated in response to conduction of the third transistor, so that an abnormal condition in any one of said control stations is signified lby the operation of the corresponding lirst `alarm indicator Vand vthe interruption in the `impedance connection normally extended between any one of said control stations and the corresponding line circuit is signiiied by the operation of the corresponding second yalarm indicator.

10. An alarm system comprising a plurality of control stations, a central station, a plurality of lines respectively extending from said control stations to said central station, each of said control stations being provided with 'a device selectively responsive to a normal condition and to an abnormal condition thereat respectively to apply corresponding normal and abnormal signals to the individual line extending thereto, a plurality ofslinecircuits at said central station and respectively associated with said lines, each of said line circuits including a transistor network selectively operative into a normal condition and into a first abnormal condition in response to the application of said respective normal and abnormal signals to the individually associated line and operative into a second abnormal condition in response to the interruption of said individually associated line, an alarm indicator at said central station, means responsive to said first abnormal operating condition of the transistor network in any one of said line circuits for selectively operating said alarm indicator to signify the abnormal condition at the corresponding one of said control stations, and means responsive to said second abnormal operating condition of the transistor network in any one of said line circuits for selectively operating said alarm indicator to signify the interruption of the corresponding one of said individual lines.

1l. An alarm system comprising a plurality of control stations, a central station, a plurality of lines respectively extending from said control stations to said central station, each of said control station being provided with a device selectively responsive to a normal condition and to an abnormal condition thereat respectively to apply corresponding normal and abnormal signals to the individual line extending thereto, a plurality of line circuits at said central station and respectively associated with said lines, each of said line circuits including a biasing network selectively operative into a normal condition and into a iirst abnormal condition in response to the application of said respective normal and abnormal signals to the individually associated line and operative into a second abnormal condition in response to the interruption of said individually associated line, each of said line circuits urther including a transistor network selectively operative into a normal condition and into a first abnormal condition and into a second abnormal condition in response to the operation of said biasing network thereof into said respective normal and rst abnormal and second abnormal conditions, an alarm indicator at said central station, means responsive to said first abnormal operating condition of the transistor network in any one of said line circuits for selectively operating said alarm indicator to signify the abnormal condition at the corresponding one of said control stations, and means responsive to said second abnormal operating condition of the transistor network in any one of said line circuits for selectively operating said alarm indicator to signify the interruption of the corresponding one of said individual lines.

l2. An alarm system comprising a plurality of alarm stations, a central station, a plurality of lines respectively extending from said alarm stations to said central station, each of said lines being arranged in a closed circuit individual thereto, each of said alarm stations being provided with a device selectively responsive to a normal condition and to an abnormal condition thereat respectively to establish corresponding normal and abnormal impedance conditions in the individual line extending thereto, opening of the circuit in any one of said lines establishing an alarm impedance condition therein, a plurality of line circuits at said central station and respectively associated with said lines, each of said line circuits including a transistor network selectively operative Pinto normal and abnormal and alarm conditions in response to the corresponding normal and abnormal and alarm impedance conditions in the individually associated line, an alarm indicator at said central station, means responsive to the abnormal operating condition of the transistor network in any one of said line circuits for selectively operating said alarm indicator to signify the abnormal condition at the corresponding one of said alarm stations, and means responsive to the alarm operating condition of the transistor network in any one of said line circuits for selectively operatiing said alarm indicator to signify the opening of the circuit in the corresponding one of said lines.

13. vThe alarm system set forth in claim l2, and further comprising means responsive to operation of said alarm indicator to signify an abnormal condition at any one of said alarm stations for locking said alarm indicator in its operated position independently of the subsequent normal or abnormal condition at the signiied alarm station, and reset means for unlocking said alarm indicator.

References Cited in the tile of this patent Bailey Sept. 16, 1952 

